The FCC process effects the transformation of heavy fractions of oil, normally mixtures of atmospheric gas oils and vacuum, into valuable products such as gas, LP gas and light olefins, the latter, inputs of processes from the synthesis of fuels and petrochemicals, and it employs acid catalysts. The FCC industrial units are designed basically with two operation stages, the first one of reaction and the second one of regeneration, and between them the catalyst circulates permanently at a constant speed. Due to the nature of the chemical reactions involved, a carbonaceous subproduct is produced, better known as coke, and this is deposited on the surface on the catalyst surface and de-activates it. The catalyst regeneration takes place by means of the combustion of the coke deposited on the catalyst, and this occurs in the regeneration stage by means of controlled air injection. Coke combustion is an exothermic reaction increasing the catalyst temperature from about 773 to 803K in the reactor up to 923K in the regenerator. The coke combustion process produces, likewise, the oxidation of the sulfur associated to the latter, and this produces in turn the corresponding sulfur oxides in relative proportions very close to thermodynamic equilibrium. These, under the occurring operation conditions, are of approximately 10% SO3 and 90% SO2. In cases where a gas purifying system is not installed, they are emitted together with the combustion gases, into the atmosphere.
Sulfur oxides, named SOx, are toxic gases referring to sulfur dioxide mixtures (SO2) and those of sulfur trioxide (SO3), and these due to the action of ultraviolet light and the humidity of the atmosphere can be transformed into sulfuric acid and generate the so-called acid rain, according to the following chemical reactions:

The extremely negative environmental of the SOx emissions has been recognized by the international scientific and industrial communities, as well as by various government agencies of most countries in the world.
The number of SOx emissions produced in the regenerator of an FCC unit is basically a function of the sulfur concentration on the load, the coke yield and the catalyst circulation rate. Approximately between 45 and 55% of the sulfur of the load is converted into H2S inside the FCC reactor, between 35 and 45% remains liquid products (gasoline, light cyclic oil and heavy oil) and between 5 and 10% is deposited on the catalyst as coke.
It is well known to persons familiar with the FCC process that control of the SOx emissions can be performed effectively and efficiently by using the catalyst itself, adequately modified with certain compounds or by means of the employment of materials known as SOx additives, added to the main catalyst inventory in relatively low (normally 10%) and controlled proportions.
Essentially, the materials that may be employed to reduce the SOx emissions should in principle be active for forming the corresponding sulfate and possess high adsorption capability. This is the case of magnesium oxide or aluminum oxide. On the other hand, thermodynamic calculations indicate that the capture of sulfur trioxide in alkaline metals is much more favorable in comparison with sulfur dioxide (Kocaefe and Karman, Cand. J. Chem. Eng., 63, 971-977 [1985]). This way, in order for a material to be employed efficiently in the FCC process, as an active agent in SOx additives, it should also possess sufficient activity to catalyze the transformation of sulfur dioxide into sulfur trioxide. Certain metals like cerium or vanadium and their oxides may be employed, due to their recognized activity as oxidation catalysts, especially suitable for oxidation from SO2 to SO3. An essential attribute for an SOx additive is that referring to its capability for detaching itself from the absorbed sulfate, and this permits regenerating the active entity and thus avoiding the saturation many materials show during the course of their use in the FCC unit. The loosening of sulfates takes place in the reaction stage upon contact of the material with the hydrocarbons found in the environment, or any other reducing agent, such as hydrogen produced as a subproduct.
The SOx additives that have been most extensively employed in the FCC units are based on magnesium aluminate spinels (MgAl2O4) containing cerium oxide and vanadium oxide. These additives are described in U.S. Pat. No. 4,790,982 to Yoo et al. and U.S. Pat. No. 4,728,635 to Bhattacharyya et al. These types of additives, however, have shown in general a relatively limited absorption capability, a tendency to lose activity quickly due to the synthetization of the active phase and a slow regeneration speed regarding the residence time there is in the reaction stage in the FCC unit. The need to use materials permitting an increase in the activity of sulfur assimilation has been recognized, both due to a greater amount and due to a higher dispersion of the active metal. One of these materials is constituted by certain anionic clays known as hydrotalcites, comprised essentially of magnesium and aluminum in a sheet structure, and the general formula of the hydrotalcites is the following: Mg6Al2(OH)16(CO3)4H2O. The presence of aluminum atoms produces negative charges in the structure, and these are compensated with interlaminar anions. What is more common, these anions are carbonates, but they may be sulfates, chlorides or others. On the other hand, the Mg/Al atomic relationship in these materials may vary between 1.7 and 5 and it is possible that bivalent or trivalent cations substitute Mg or Al.
The hydrotalcite synthesis is generally performed by means of a co-precipitation of metal salts. A classic preparation consists in mixing an aqueous solution of magnesium and aluminum salts, for example, nitrates or chlorides, and add it to a sodium carbonate solution under continuous stirring. The precipitate formed is submitted to heating for several hours at temperatures between 333 and 473K.
In U.S. Pat. No. 5,750,020 (1998), to Bhattacharyya et al., granted to AMOCO, Co., a collapsed hydrotalcite composition is described, and this may be obtained by calcining a mixed sheet hydroxide having monometallate anions on the interlaminar region. This collapsed composition is comprised substantially of microcrystals represented collectively by means of the formula: M2m2+Al2−pMp3+TrO7+r.s, where M2+ is a divalent metal, M3+ is a trivalent metal and T is vanadium, tungsten or molybdenum. The little crystals are so small they cannot be detected by means of conventional X-ray diffraction techniques; however, high resolution electronic microscopy reveals that a considerable portion of the little microcrystals is comprised of a solid solution of molecularly disperse aluminum oxide in the crystalline structure of the divalent metal monoxide. Another portion of the little microcrystals is constituted by the spinel phase. The collapsed composition functions to absorb sulfur oxide and has a comparatively high absorption capability and comparatively high absorption and desorption speeds, and also functions as a catalyst for nitrogen oxide reduction.
In U.S. Pat. No. 5,785,938 (1998) of Pinnavaia T. J. et al. a process to remove and later capture sulfur oxides from gas currents is described, particularly those effluents from the energy generating plants through coal burning, by using double metal hydroxides containing Ni, as recyclable absorbents. The absorbing compositions contain metal components forming stable sulfites and sulfates at a temperature, but they are decomposed at a higher temperature to regenerate the absorbing material.
In U.S. Pat. No. 5,928,496 (1999), a process for the sorption of sulfur oxide by using hydrotalcite type materials, like contact solids, is described. Hydrotalcite type materials are stable when transformed to the crystalline structure of the oxide and are essentially reversible in an exchange of anions. These materials, made up of large crystal sheets and with a high SOx sorption capacity, are benefited in their sorption properties when modifying them with certain species of organic acid anions, which modify the hydrotalcite/brucite structure.
In U.S. Pat. No. 6,028,023 (2000) of Vierheilig A. A., assigned to Bulldog Technologies Inc., a process for the preparation of hydrotalcite type compounds is described, where a non-hydrotalcite type (or hydrotalcite type) compound is thermally treated and afterwards hydrated to form hydrotalcite type compounds, thus obtaining properties (such as: greater hardness, and/or density) differing from those of hydrotalcite type compounds synthesized by methods reported in the prior state-of-the-art, where non-hydrotalcite type materials (or hydrotalcite type compounds) are not thermally treated in a similar manner or hydrated for forming such hydrotalcite type compounds.
Hydrotalcite type compounds, also called: double or triple laminar metal hydroxides or anionic clays, are materials with laminar structure represented by the following formula:[M(II)1−xM(II)x(OH)2](Ax).mH2Owhere:                M(II), represents a divalent cation.        M(III), represents a trivalent cation.        A, represents any anion.        
Metals form octahedrons joining their edges to form positively charged bidimensional sheets. The positive residual charge is originated in the metal or trivalent cation. To neutralize this residual charge, it is necessary for there to be anions located among the sheets, which may be exchanged by others of chemical composition different from the initial one. Due to the polar and hydrophilic nature of the sheets and depending on the space vacated by the anions, a certain amount of water may be accommodated among the anions and among the sheets.
Specialists in this field will notice that anionic clays are commonly called “mixed metal hydroxides.” This expression is derived from the fact that, such as was indicated previously, the positively charged sheets of the metal hydroxides may contain two diverse metal cations in different oxidation states (for example, Mg2+, Al3+, and so forth). Additionally, and given that the X-ray diffraction patterns of so many anionic clays are similar to the mineral named hydrotalcite, Mg6Al2(OH)16(CO3).4H2O, it is common for them to be called “hydrotalcite type compounds.” This term has been broadly used in the literature of scientific articles and patents during many years (See, for example, Pausch “Synthesis of Disordered and Al-Rich Hydrotalcite-Like Compounds”, Clays and Clay Minerals, Vol. 14, 507-510 (1986). Such compounds are frequently referred to as “anionic clays.” In fact, the terms “anionic clays”, “mixed metal hydroxides” and “hydrotalcite type compounds” are very closely related, if not used indistinctly. On the other hand, U.S. Pat. No. 5,399,329 contains the following sentence: The term “hydrotalcite type” is recognized in its field, defined and used consistently with the use comprised in the monograph type revision articles referred to by Cavani et al.” Therefore, and for the purposes of this patent application, the compound term “hydrotalcite type” will be used (unless otherwise indicated) with the understanding that this term should be considered inclusive of anionic clays; the hydrotalcite itself, as well as any member of the class of materials known as “hydrotalcite-type compounds.” Moreover, the term “hydrotalcite type” will be abbreviated as “HT” in the present patent application.
It is also known that HT's are decomposed in a predictable manner when heated without exceeding certain temperatures. The resulting materials from decomposition may be re-hydrated (and optionally, re-supplied with certain anions, e.g. CO3═, removed during heating); and likewise, the original HT or a similar one may be reproduced. Products from the decomposition of such heating are frequently referred to as “collapsed” or “meta-stable” hydrotalcite type compounds. If these collapsed or meta-stable materials, however, are heated at certain temperatures higher than 1023K, then the products of the decomposition of such HT's cannot be re-hydrated and/or reconstituted anymore. Therefore, they cannot rebuild the original HT.
Such thermal decomposition process of hydrotalcite type compounds has been studied in detail in academic and patent literature. For example, Miyata, “Physico-Chemical Properties of Synthetic Hydrotalcites in Relation to Composition”, Clays and Clay Minerals, Vol. 28, 50-56 (1980).
One of the most difficult problems to solve when wishing to prepare ternary or multimetallic hydrotalcites, in which the three or more cations have actually been incorporated into the laminar structure, is the fact that depending on the chemical nature of the cation, its precipitation speed may be different from that of the other cations, assuming a constant pH. If the precipitation speeds are very different among themselves, non-hydrotalcite separate crystallographic phases will be obtained, that is, the three cations may not be incorporated uniformly into the hydrotalcite layers. If the situation is complicated when we have a binary system (two cations) due to the aforementioned reasons, the situation becomes even more complicated when we have three or more. Therefore, it is difficult to obtain ternary hydrotalcites whose crystallographic phases show that they are pure within an ample range of compositions of the three cations. Even so, there are some reports demonstrating it is possible to obtain them (J. Sánchez-Valente, Doctoral Thesis, “Synthèse et Characterisation des solides du type hydrotalcite et leurs applications en Catalyse” IRC, Francia 1999., M. Sánchez-Cantú, Bachelor's Degree Thesis IMP, March 2002).
In scientific literature, there are some examples of ternary hydrotalcites, that is, those made up in their laminar composition of three different cations. However, these examples are scarce, since a greater effort has been devoted to the study of binary hydrotalcites. For example, Kooli et al. (1995) have reported the preparation of Ni—Al—Cr and Ni—Al—Fe hydrotalcites containing carbonate anions in their interlaminar region. These ternary hydrotalcites, where the traditional trivalent cation (Al3+) is partially substituted by Fe3+ or Cr3+ cations, were obtained by means of a co-precipitation with sodium carbonate and sodium hydroxide at a pH=10, at 333K, followed by a hydrothermal treatment at 423K. The ternary hydrotalcites were obtained in pure crystallographic phases when the following atomic relationship ranks were maintained between the cations: 3.6≦Ni/(Cr+Al)≦6.8 or 2.3≦Ni/(Fe+Al)≦9.9; beyond these ranks other non-hydrotalcite phases were detected.
The foregoing group of authors (F. Kooli et al., 1995) have also described the preparation of ternary hydrotalcites where the divalent cation is partially substituted by others, for example Mg—Zn—Al and Zn—Cu—Al. In this procedure, the solids are obtained by means of co-precipitation with sodium carbonate and sodium hydroxide at 333K and pH=10. The atomic relationships between which it is possible to obtain crystallographically pure hydrotalcite phases were the following: 1.3≦(Mg+Zn)/Al≦5.9 and 2.7≦(Zn+Cu)/Al≦3.1; however, even within these ranks there were occasional non-hydrotalcite phases.
Cu—Ni—Al ternary hydrotalcites, where the divalent cation is partially substituted by another one and with carbonate and nitrate anions in the interlaminar region, have been obtained by means of co-precipitation with an aqueous solution of NaOH, at pH=10 and room temperature, as is reported by A. Alejandre et al. (2001). The precipitate obtained was aged at 343K during 2 h. However, in the case of these ternary hydrotalcites, where Cu is from 0.11 to 0.32, Ni is 0.15 to 0.36 and Al is constant and equal to 3, that is, in order to maintain a divalent/trivalent cation atomic relationship=0.16, their diffraction patterns always show different phases from those of hydrotalcite, or mixtures of such phases.